In order for a woodwind instrument to have accurate sound, it is critical that the tone hole sealing pad make a complete seal flat against the tone hole rim. Since the tone hole rims are almost never perfectly flat, the pads are almost never perfectly flat, and the cup that holds the pad is almost never perfectly flat, the pads must be installed in the cup using shims behind the pad in order to get as close to a sealing fit as possible. The installation is made more difficult by the fact that the cup approaches the tone hole from an angle, making this shimming more guess work than science.
There have been a number of techniques developed in attempts to provide relatively easy installation of pads that properly seal around the entire circumference of the tone hole rim. One method contemplates floating the pads into place on a liquid placed in the cup, which then hardens to serve as a permanent shim between the cup and the pad. In this process, while the pad is floating on the liquid, the installer pushes the pad against the tone hole rim by moving the cup down towards the rim. Ideally, the pad will seat flat against the rim and the liquid will harden, securing the pad in the position necessary in the cup to provide a flat fit of the pad against the rim. Typically, shellac is used as the liquid for floating the pad; the shellac is melted by the application of heat to the cup. Various other adhesives have also been used.
This floating technique suffers from a number of problems. The larger pads for the woodwind instruments having larger tone holes are inherently more flexible than the smaller pads and thus are even more likely not to be flat. Since the traditional floating process relies primarily on pads being flat and rigid, the process does not work well for these large pads. Further, the pad installer must very carefully push the cup against the tone hole rim while seating the pad in the floating process, since there is really no effective way to pull the pad out of the cup once it is pushed into the cup. Sometimes the installer can stick a pin in the side of the pad and lever it back out of the cup, but this procedure can damage the pad skin, is imprecise, and can sometimes leave air bubbles behind the pad which allow the pad to collapse at this point in use. One method that has been developed to move the pad around in the cup while it is being installed in such a manner is to place a shim or slick between the pad and the cup under one section of the pad in order to compensate for some imprecision in the floating process. However, this requires that the operator perform educated guesswork, which is fraught with problems and also requires an experienced and thus expensive installer.
For pads that are held in a cup with a screw and a washer, the pads are typically shimmed with extremely thin pieces of paper placed between the pad and the cup to push the pad forward so that it lies flat against the tone hole rim. This installation is typically accomplished by the installer first applying a single paper shim cut in a desired shape to match as closely as possible the pad to the rim. Then, the pad is pressed against the rim using the instrument keys. The installer can then use a feeler gauge or a light source placed inside the instrument tube to find gaps between the pad and the rim. The installer identifies and marks the location of the gaps, removes the key from the instrument, removes the pad from the cup, and then adds one or more shims to make up for the gaps. The pad is then placed back into the cup, the key is reassembled onto the instrument, and the pad closure is tested again with a feeler gauge or light source. The shimming process is repeated as often as necessary to accomplish the desired seating.
This process suffers from the drawbacks that it is time consuming and depends on the judgment of the installer regarding the location and thickness of paper shims. Since this judgment is experiential, the installer must be skilled, and the process is therefore expensive. Further, the cup surfaces typically are not flat. Accordingly, over time the paper or plastic shims can collapse into the curve of the cup, destroying the flatness that is accomplished with the shims. Another problem with this installation technique is that it requires frequent replacement of the metal screws, washers and/or bushing which hold the pad in place. Since it is nearly impossible to accomplish such replacement in the same way twice, the installer may conclude that a pad is leaking because of improper shimming when in fact the placement or tension of the metal pad holder has altered the position of the pad. Finally, the metal pad holders amplify the noise of the pad striking the tone hole rim and are thus undesirable in good quality instruments.
A third pad installation technique for felt pads contemplates securing the pads in the cups using metal or plastic pad holders that are clamped in place. Steam is then circulated through the body of the instrument to set the tone hole rim impression in the pad. However, this setting is impermanent and thus the pad will have a tendency to return at least partially to its original position, causing the pads to leak. Further, these felt pads are thick and fluffy thus have a spongy feel to the player which is undesirable because the player does not know exactly when the pad is in contact with the tone hole rim. Additionally, pads which can take a deep impression of the tone hole rim create more surface on which the pad can stick to the tone hole rim, making play difficult. The metal pad holders amplify the noise of the pad striking the tone hole rim, making them undesirable. Finally, since the technique requires steam, it cannot be used on instruments made of wood.